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But school teachers say they tried to work with the student to care for the young cat.
Fifteen-year-old student Nathan Gunn earlier pulled the shivering, muddy feline out of the drain at Darfield High School after hearing it meowing for help.
Wrapping it in his hoodie to warm it up, Gunn kept the kitten with him during morning classes, before getting his mum's permission to take it to the vet for a check-up during the lunchtime break.
But when he went to the school's office to sign out and leave the grounds, he claimed he was refused.
Darfield principal James Morris and a deputy principal instead simply told Gunn to put the kitten back in the drain, he claimed.
Gunn tried to plead his case, saying the kitten was soaked through and shivering, but was eventually left disgusted and angry when the school didn't back down, he said.
"They were encouraging us to kill it pretty much - let it suffer or die of dehydration or cold," he said.
Gunn's mother has now put in a formal complaint with the school.
However, Darfield principal Morris said he offered students three options to care for the kitten.
Gunn said he had earlier become concerned about the kitten after seeing a group of younger girls huddled around the drain.
They had been drawn by the kitten's screams for help that could be heard from at least 30m away.
They took the kitten to school's teachers to ask what could be done, but were told to put the young cat back in the drain, Gunn claimed.
"I thought that's disgusting," he said.
"So I went and grabbed it out of the drain and wrapped it up in my hoodie and went through my day holding the cat."
Covered in mud, the kitten shivered for about three hours, leading Gunn to become concerned it needed a check-up.
Being unable to take the young male cat to the vet during lunch, Gunn and his girlfriend's family eventually got it checked up after school.
"He is quite healthy actually, and just needs all the shots and medication, like worming," Gunn said.
However, the vets also warned Gunn the kitten needed carers to look after it because it was so young.
Being barely able to walk and not yet able to eat solid food, the kitten would probably be put down if it was brought back to the vets without a home, they warned.
Luckily, Gunn and his girlfriend put up a Facebook post calling for someone to look after the cat and were inundated with responses.
Despite the happy ending, Gunn believes his teachers should "get a turn-up" over the incident.
"They can't just tell their students to go throw it back in the drain ... and let it die," he said.